1. Field of the Invention
In general, the present invention relates to cellular telephone networks, and more particularly to using fuzzy logic to control handovers in such systems.
2. Statement of Related Art
A cellular telephone network is a system that provides radio communication to and from mobile phones over a large geographical area. Cellular networks generally consist of a plurality of adjoining geographic regions, subdivided into cells. A portion of a prior art cellular telephone network is depicted in FIG. 1.
Each geographic region, commonly referred to as a cell 10, has a base transceiver station 12, which can communicate via radio waves to and from a mobile telephone, which is not shown in FIG. 1. Mobile telephones are often used inside an automobile 22. Each base transceiver station is also connected to a base station controller 16 by a group of land-based telephone lines, commonly referred to as a trunk 14. Each base station controller 16 is typically connected to several base transceiver stations 12, with each base transceiver station 12 serving a different cell 10. Each base station controller 16 is connected by a land-based trunk 18 to a mobile switching center 20. A mobile switching center 20 is typically connected to several base station controllers 16. Finally, a cellular network typically contains several mobile switching centers 20, each mobile switching center 20 is typically connected to several other mobile switching centers 20 and is connected to several base station controllers 16, and each base station controller 16 is typically connected to several base transceiver stations 12.
During a cellular call, a mobile phone communicates with the base transceiver station 12 by using a particular frequency of radio waves. That frequency is pre-determined by the base station controller 16 for the cell 10 in which the mobile phone is located. Often, mobile phones are used in moving automobiles 22 and as the mobile phone moves away from the base transceiver station 12 handling the call, typically the mobile phone gets closer to one or more other base station transceivers 12 and the quality of the transmission between the mobile phone and the base transceiver station 12 handling the call tends to deteriorate. If the quality of transmission decreases significantly, the call must be "handed over" to a different base transceiver station 12 so that the call can continue between the mobile phone and the base transceiver station to which the call is handed over.
Although the mobile phone is initially tuned to a pre-assigned frequency for the call, the mobile phone also monitors other frequencies during a call. The frequencies being monitored may belong to base transceiver stations 12 other than the base transceiver station 12 handling the call. Periodically, the mobile phone transmits, to the base transceiver station 12 handling the call, measurements related to the signal quality for the frequencies that it is monitoring, including the frequency being used for the mobile call in progress.
The two directions in which the mobile phone and a base transceiver station communicate with each other via radio waves are referred to as a down-link and an up-link. The down-link refers to transmissions from the base transceiver station 12 to the mobile, and the up-link refers to transmissions from the mobile phone to the base transceiver station 12.
The mobile phone measures the down-link signal strength, the down-link signal quality, and the battery power being used by the mobile phone to transmit radio waves to the base transceiver station 12. During a mobile call, these measurements are transmitted to the base transceiver station 12, which in turn sends them to the base station controller 16. The base transceiver station 12 measures the up-link signal strength, the up-link signal quality, and the distance between the mobile phone 30 and the base transceiver station 12. The base transceiver station 12 then relays these measurements to the base station controller 16, which determines whether to hand over the call based on the measurements taken by the mobile phone 30, the measurements taken by the base transceiver station 12, the call duration and the rate at which handovers are being performed for a particular call. Call duration and rate of handover are maintained by the base station controller 16.
A base station controller 16 and the base transceiver stations 12 that are connected to the base station controller 16 are referred to as a base station system 24. If a call is handed over from one base transceiver station 12 to another base transceiver station 12 in the same base station system 24, the handover is referred to as an intra-base-station-system handover. If a call is handed over to a base transceiver station 12 in a different base station system 24 that is connected to the same mobile switching center 20 as the first base station controller 12, then the handover is referred to as an intra-mobile-switching-center handover. If a call is handed over to a base transceiver station 12 in a different base station system 24 that is connected to a different mobile switching center 20 than the first base station system's 24 mobile switching center 20, then the handover is referred to as an inter-mobile-switching-center handover.
Excessive handovers are undesirable. When a handover must be performed, the telephone system resources needed for the call must be reserved on another base transceiver station 12, which might be contained within a different base station system 24 and which might be connected to another mobile switching center 20. Inter-mobile-switching-center handovers require setting up voice paths on a trunk between the two mobile switching centers and on a trunk 18 between the new mobile switching center 20 and the new base station controller 16. Setting up these voice paths, requires extensive signaling between the mobile switching centers 20 and between the base station controllers 16 and the mobile switching centers 20.
The boundaries of adjacent cells 10 are usually not well-defined. Therefore, when a mobile phone, for instance, in a moving automobile 22, moves close to the boundaries between geographically adjacent cells, handover control mechanisms using conventional crisp logic often cause unnecessary handovers.
Techniques for determining when to perform handovers are known in the art, and make handover decisions based upon the distance between the mobile phone 30 and the base transceiver station 12 handling the call, the signal quality of the up-link and of the down-link (collectively referred to as "quality"), the up-link and the down-link signal levels (collectively referred to as "level"), in decibels, the power being used to transmit radio waves over the up-link and the down-link (collectively referred to as "power"), and the number of calls being processed by the base transceiver station.
Existing mechanisms for controlling handovers in cellular telephone networks use crisp, as opposed to fuzzy, logic. Using crisp logic in determining whether to perform a handover suffers from the drawback of causing very definite triggers with no way to account for inaccurate data related to the quality of the up-link and down-link radio transmissions . Further, handover is a very complex mechanism. Often, handovers are triggered by little known causes. Handovers can also cause cellular calls to be dropped.
In certain situations, rather than simply deciding whether or not to handover a call, it is desirable to adjust the power being used to transmit over the up-link or the down-link, or both, in order to improve the quality of the received signal.
Crisp logic is based on set theory where a member either belongs to a set or it does not. For example {1,2,3, -1,-2,-3} belong to the set called Integers. {1.2,1.3,1.5} belong to the set of real numbers, but not to the set of integers. For instance, the color of a ball could be in the set {red, blue, green}. However, when the ball is somewhat red, that concept cannot be quantified by discrete mathematical set theory. Fuzzy logic or Fuzzy Set Theory solves this problem by allowing the members of a set to have certain grades of membership that will quantify fuzzy criteria such as not very red, somewhat red, fairly red, very red, etc.
Fuzzy set theory was first proposed by L. A. Zadeh in the 1960s. Since then it has been used in numerous applications such as washing machines, cameras, artificial intelligence, linguistics, economics, filters, databases retrieval systems. Although non-linear systems can be very complicated to model using mathematics base on traditional set theory, fuzzy logic provides a simplified approach to modeling such systems.
It is therefore an object of the present invention to use fuzzy logic to determine whether a handover should be performed and whether the radio transmission power should be increased or decreased in order to more reliably control when handovers are performed, thereby reducing the system resources required to process a particular volume of calls, improving signal quality for calls on the network, and reducing the number of dropped calls.